Cepaea nemoralis is a species of land snail distributed from northern, western, and central Europe. The species has been the subject of many studies into the nature of polymorphism due to the variety of shell colours - ranging from extremely pale, almost white, yellow, pink and brown. The banding patterns of the shells also vary from no bands, up to six, and vary in colour and width. The shell polymorphisms are simply inherited, making them ideal for observing in polymorphism studies.
An additional reason for studying this organism as opposed to humans is the generation time – Cepaea nemoralis have a much shorter life span so data from several generations can be easily observed. There are also ethical reasons such as the ability to include dead snails within the study. In a typical snail’s lifetime it will not travel much more than 20 metres, so in order to sample the species a much smaller area is needed – the equivalent in humans would be sampling across continents. The organism is also easy to collect because they are small and slow moving.
To ensure that snails from other populations are not included in the experiment, all of the samples will be taken in one area which has several different habitats. Six samples will be taken over two of these habitats – grass and woodland – and will all be at least 20 metres apart. This number of samples will allow us to make comparisons to show whether it is selection is having a great effect on the variation in snail shell colour and pattern. In each sample at least 30 snails should be collected as a larger sample will minimise the effect of sampling error. As many variables as possible will be eliminated, for example all the samples will be taken at the same altitude, facing the same aspect, and on the same day (this should mean the weather conditions remain constant). The samples will be taken as far away from the boundary between the environments as possible to ensure there is no ambiguity between the grass and woodland areas.
If a higher proportion of dark Cepaea nemoralis is found in dark areas and a higher proportion of light Cepaea nemoralis is found in light areas, the hypothesis will be supported. This needs to be seen consistently across all the samples in each environment so that there is no significant difference between the proportions in the same environment. The hypothesis will not be supported if the same proportions of polymorphisms are seen across all of the samples, regardless of environment, or if completely random proportions with no pattern are found. If only one polymorphism is observed then it has been fixed, but it will not be known whether this is due to genetic drift or selection. It is hoped that the study will show the effect of selection pressures acting on snail polymorphisms between populations in different environments. Which selection pressures are acting on the polymorphisms cannot be determined by the sampling, but theories can be developed from the results.
Introduction
Cepaea nemoralis is a species of land snail distributed from northern, western, and central Europe. The species has been the subject of many studies into the nature of polymorphism due to the variety of shell colours - ranging from extremely pale, almost white, yellow, pink and brown. The banding patterns of the shells also vary from no bands, up to six, and vary in colour and width. The shell polymorphisms are simply inherited, making them ideal for observing in polymorphism studies.
An additional reason for studying this organism as opposed to humans is the generation time – Cepaea nemoralis have a much shorter life span so data from several generations can be easily observed. There are also ethical reasons such as the ability to include dead snails within the study. In a typical snail’s lifetime it will not travel much more than 20 metres, so in order to sample the species a much smaller area is needed – the equivalent in humans would be sampling across continents. The organism is also easy to collect because they are small and slow moving.
To ensure that snails from other populations are not included in the experiment, all of the samples will be taken in one area which has several different habitats. Six samples will be taken over two of these habitats – grass and woodland – and will all be at least 20 metres apart. This number of samples will allow us to make comparisons to show whether it is selection is having a great effect on the variation in snail shell colour and pattern. In each sample at least 30 snails should be collected as a larger sample will minimise the effect of sampling error. As many variables as possible will be eliminated, for example all the samples will be taken at the same altitude, facing the same aspect, and on the same day (this should mean the weather conditions remain constant). The samples will be taken as far away from the boundary between the environments as possible to ensure there is no ambiguity between the grass and woodland areas.
If a higher proportion of dark Cepaea nemoralis is found in dark areas and a higher proportion of light Cepaea nemoralis is found in light areas, the hypothesis will be supported. This needs to be seen consistently across all the samples in each environment so that there is no significant difference between the proportions in the same environment. The hypothesis will not be supported if the same proportions of polymorphisms are seen across all of the samples, regardless of environment, or if completely random proportions with no pattern are found. If only one polymorphism is observed then it has been fixed, but it will not be known whether this is due to genetic drift or selection.
It is hoped that the study will show the effect of selection pressures acting on snail polymorphisms between populations in different environments. Which selection pressures are acting on the polymorphisms cannot be determined by the sampling, but theories can be developed from the results.